White is the most common colour for plastics. It is most often achieved by using titanium dioxide pigment, which tends to have a yellow undertone. This unwanted yellowness can be exaggerated by extending the white pigment with less pure fillers, or by a colour shift in the resin during processing.

　　Colour correction is a term frequently used to describe the effect observed when low concentrations of Ultramarine Blue and Violet or Manganese Violet pigments are added to media which possess an inherent yellowish tint. The improvement in apparent whiteness that can be achieved in this way is dramatic. “Whiter than white” shades can be produced and, with transparent materials, the apparent clarity can be greatly enhanced.

　　Ultramarine and Manganese Violet pigments are ideal for colour correction. Both pigment types strongly absorb yellow light.

　　It is the yellowish tinge which is often responsible for giving an unattractive appearance to many white or clear plastics. The effect of adding an appropriate amount of a colour correcting pigment is to give a more neutral shade. Although the overall brightness may be slightly reduced, this pigment addition produces a shade which is more aesthetically pleasing.

　　Ultramarine pigments can be used very successfully in combination with optical brighteners to achieve a synergistic effect. Ultramarines exhibit low absorption of ultra-violet light and, therefore, do not interfere with the mechanism by which optical brighteners operate. As a consequence, the two products together give optimum results.

　　Both Ultramarine and Manganese Violet are transparent, so can be used to absorb unwanted yellowness in white articles and also in clear polymers such as PVC.

　　Colour correction is a subjective effect based on the aesthetic preference of the observer. Ultramarine Blue is used to produce what may be called a ‘cold’ white, whereas the violet pigments produce ‘warm’ whites. Very often, combinations of the blue and violet pigments are used to produce the desired effect. Where the initial degree of yellowness is high, an increase in the ratio of violet to blue can significantly improve the final result.

　　The precise pigment concentration needed to produce the desired level of colour correction depends on the initial degree of yellowness. Generally it is not necessary to use concentrations higher than 0.05% and often addition levels are much lower.

　　The moderate colour strength of Ultramarines and Manganese Violet is a further advantage of these pigments. This enables accurate metering or weighing at the low pigment concentrations used for colour correction.

　　After titanium dioxide, Carbon Black is the most commonly used pigment for colouring plastics. When used as the sole pigment it gives a black colour, while combinations with titanium dioxide results in grey shades in a range of depths. It may seem strange to think of black and grey as “colours” but that is exactly what they are. Like any other colour they will have subtle tones and undertones that add range to the shade.

　　Higher strength Carbon Black pigments are invariably brown in undertone. In many situations this is undesirable. There are many instances where fashion or compatibility with related objects requires a bluer undertone grey than is possible with a high strength Carbon Black alone. For example, car and aircraft interiors, computer housings, office equipment etc. A blue undertone grey is effectively achieved by the addition of Ultramarine Blue.

　　As in the correction of whites, Ultramarine strongly absorbs the yellowness responsible for the brown undertone of these Carbon Blacks. Because black is such a strong colour it requires a higher addition rate of Ultramarine to have a noticeable effect.